This invention relates to tongs that support glass sheets during a press bending operation, particularly one in which a heat softened glass sheet has a sharp bend imparted thereto by pressing between complementary molds immediately preceded by localized heating along the intended line of the sharp bend. Electric resistance heating provides the localized heat along the intended line of the sharp bend immediately prior to the pressing step.
Vertical press bending is well known in the art, as exemplified by U.S. Pat. No. 3,367,764 to S. L. Seymour. Prior art press bending processes, however, have been limited as to the sharpness of curvature which may be imparted to the glass. A minimum radius on the order of about 9 inches (23 centimeters) or more is typical of commercial press bending installations. It would be desirable to produce "V" bends having radii less than 8 inches, preferably on the order of 4 inches (10 centimeters) or less.
The production of sharp "V" bends has heretofore been most commonly carried out by gravity sag bending processes, wherein glass sheets are placed on contoured outline molds in a generally horizontal orientation and heated to the softening point of the glass so that the glass sags to conform to the shape of the mold. In order to cause such a bend to be concentrated along a line to form a "V" bend, localized heat is applied along that line by electric resistance heating through a wire or ribbon held close to or in contact with the glass or through an electroconductive substance applied to the glass. The use of a heated wire or ribbon is disclosed in U.S. Pat. No. 3,795,501 to R. A. Jansson and T. J. Reese; U.S. Pat. No. 3,795,570 to R. A. Jansson and D. L. Thomas; U.S. Pat. No. 3,847,586 to T. J. Reese, G. R. Claassen, and M. W. Tobin; U.S. Pat. No. 2,111,392 to J. H. Galey; U.S. Pat. No. 2,176,999 to R. A. Miller; U.S. Pat. No. 2,215,228 to J. G. Oliver; U.S. Pat. No. 3,248,195 to J. G. Golightly and H. E. McKelvey; and U.S. Pat. No. 2,999,338 to R. L. Richardson. The use of electroconductive stripes applied to the glass surfaces is disclosed in U.S. Pat. No. 3,762,903 to H. E. Hamilton; U.S. Pat. No. 3,762,904 to H. E. Hamilton, R. E. Bamford, and P. Pastorek; U.S. Pat. No. 3,879,184 to H. E. Hamilton and I. L. Soreghy; and U.S. Pat. No. 3,865,680 to T. J. Reese and H. S. Koontz.
Prior to this invention it has been proposed to apply localized heat to glass sheets in a press bending operation, but not in a manner compatible with forming "V" bends. One such proposal is disclosed in Canadian Patent No. 799,907 to R. E. Richardson where the leading edge of each glass sheet is heated in the press bending station in order to equalize the temperature of the sheets from their leading edge to their trailing edge. The heat is provided by radiant heaters aimed at the general region of the sheet near the leading edge. U.S. Pat. No. 3,333,935 to C. E. Valchar and S. J. Mrozinski shows a press bending method wherein the lower portion of each glass sheet is given additional heat by means of a pair of gas burners in order to enable that portion of the sheet to be bent to a sharper curvature. Such an arrangement, however, does not produce the narrow, precisely located lines of heating which are required for making "V" bends. Another use of localized heat during press bending is disclosed in U.S. Pat. No. 3,960,535 to H. E. Hamilton and W. W. Oelke, which discloses the application of heat with gas burners to the portion of glass sheets around holes in the sheets during press bending. The object is to maintain equalized temperature conditions in the glass, and therefore has very little in common with the production of "V" bends. U.S. Pat. No. 3,854,920 to Kay and Pickard shows a press bending mold with a plurality of elongated electrical heating elements extending along vertical lines in the mold, but the heating elements are beneath the surface of the mold since their purpose is to warm the entire mold face, not to impart localized heat to the glass.
In copending application Ser. No. 756,209 of Joseph B. Kelly and Kenneth A. Gibson filed Jan. 3, 1977, for forming "V" bends in glass sheets by press bending, a method and apparatus is disclosed that entails the application of a stripe of electroconductive material onto the surface of a glass sheet along a line corresponding to the intended location for a "V" bend. The sheet, with the strips in place, is heated as a whole to a temperature suitable for bending in the conventional manner by passing the sheet through a furnace. Upon leaving the furnace, the sheet is stopped between spaced, opposing, complementary press members, but before the press members engage the glass sheet, a pair of electrodes is moved into contact with the electroconductive stripe at opposite edges of the glass sheet so as to complete a circuit through the electroconductive stripe. A high voltage current is passed along the line defined by the stripe for a few seconds so as to heat the glass along the line to a temperature higher than that of the body of the glass. The electrodes are then withdrawn and the pressing commences immediately in the usual manner. The localized heating along the line of the "V" bend enables press bending to produce a radius of curvature well below 8 inches (20 centimeters).
Usually, glass sheets are suspended from metal tongs which, in turn, are connected to metal carriages that are provided with rails that ride on grounded rollers. Glass sheets are heated to a temperature at which the glass becomes electroconductive in order to be shaped by press bending. Therefore, the application of electrical energy along the line desired for sharp bending results in a leakage of electricity through the tongs and the tong supporting carriages to ground. Such leakage reduces the effectiveness of the voltage applied along the line desired for sharp bending and results in a bend of less sharpness than desired and an uncontrolled temperature pattern in the portion of the glass causing undesired added heating between the applied electroconductive line and the glass supporting tongs.
This additional heating causes all sorts of problems in the glass. The tongs indent into the glass to a greater extent than desired because the additional heat softens the glass in this region. Therefore, more severe localized distortion and marking of the glass results in the vicinity of the locations where the glass engaging elements of the tongs grip the glass than when no current is applied across the tong-gripped glass sheet.
U.S. Pat. No. 3,340,039 to Marceau discloses a glass sheet gripped by metal tongs which are connected by metal bars to metal carriages provided with metal wheels that ride along metal rails. This patent provides a resilient or shock-absorbing strip that is placed on the rails upon which the carriage wheels travel. The resilient strip is preferably rubber. While some rubber compositions do not conduct electricity, and apparatus disclosed in the Marceau patent may avoid the possibility of a direct grounding circuit from an electroconductive glass sheet through glass gripping tongs, connecting bars and carriage and wheels to a rail, the apparatus disclosed in this patent is such that if a high enough voltage were used to cause localized heating of the glass by electrical resistance, no provision is made to avoid electrical arcing between the wheels and track on which the wheels ride.
U.S. Pat. No. 3,363,930 to Webb discloses tongs having glass engaging elements composed of a fired ceramic composition having a high aluminum oxide content having a greater coefficient of friction with glass than stainless steel throughout the temperature range at which the glass is thermally processed to produce fabricated glass products. This patent also discloses several other embodiments of glass contacting elements composed of other ceramic materials having the requisite properties of greater hardness and higher tensile strength than pressed asbestos and substantially less thermal conductivity and a substantially lower thermal capacity than that of stainless steel. Some of these materials are inherently electrical insulator materials. However, the size of the glass contacting elements is relatively small so that if a high voltage were applied to an electroconductive glass sheet carried by tongs provided with the Webb glass engaging elements, arcing would not be avoided, even though there is a possibility that some of these materials listed on Webb would inherently have the electrical insulator properties whose characteristics were not appreciated prior to the present invention.